In recent years, water-absorbent resins have been widely used in the fields of hygienic materials such as sanitary articles and disposable diapers.
For water-absorbent resins as described above, crosslinked products of partially neutralized polymers of acrylic acid are preferred because they have many advantages, including the followings: they have excellent water-absorption performance; their raw materials such as acrylic acid has easy industrial availability, and therefore they can be manufactured with stable quality and low cost; and they show no shortcomings in which, for example, decomposition and degradation are likely to occur.
Examples of the desirable property of a water-absorbent resin in hygienic materials such as sanitary articles and disposable diapers include a high water-absorption capacity and an excellent water-absorption rate. However, for example, since a water-retention capacity and a water-absorption rate have a conflicting relationship, it is difficult to satisfy a balance between these properties.
As technologies for enhancing the properties of the water-absorbent resin suitably used for hygienic materials, for example, the following technologies are known: a method of performing reverse phase suspension polymerization using specific amounts of specific polymer protective colloid and surfactant (see Patent Document 1); a method of performing reverse phase suspension polymerization in multiple steps of two or more steps (see Patent Document 2); a method of performing reverse phase suspension polymerization under the coexistence of β-1,3-glucans to obtain a water-absorbent resin, and furthermore adding a crosslinking agent to the obtained water-absorbent resin to perform a crosslinking reaction (see Patent Document 3); a method of performing reverse phase suspension polymerization using a specific amount of persulfate using as a polymerization initiator (see Patent Document 4); and a method of performing aqueous solution polymerization in the presence of a phosphorous acid and/or a salt thereof to obtain a water-absorbent resin precursor, thereafter mixing the water-absorbent resin precursor and a surface-crosslinking agent and heating them (see Patent Document 5).
However, the water-absorbent resins obtained in these methods do not necessarily satisfy the high water-absorption capacity and the excellent water-absorption rate described above, and there are still improvements to be made.
In an absorbent material containing a water-absorbent resin, when the water-absorbent resin in which the diffusion property of a member to be absorbed is low is used, in the vicinity of the position of supply of a liquid to be absorbed, the water-absorbent resin, locally absorbs the liquid to be absorbed, and the swelled water-absorbent resin becomes dense, with the result that the blocking of the liquid often occurs. In this case, since the gelled water-absorbent resin further inhibits the diffusion property, the amount of re-wet of liquid to be absorbed tends to be increased.